A single-color or multi-color prepress proof suitable for proof printing, etc., has recently been increasingly produced. The prepress proof is produced by imagewise exposing an image-forming material comprising a substrate and a photosensitive layer formed on one surface of the substrate and then transferring an image to an image receptor, such as paper, under heat, generally around 100.degree. C. The image-forming material is formed of a substrate, a photosensitive layer formed on one surface of the substrate and a protection layer. As the substrate and the protection layer, a thin film or sheet formed from a material such as cellulose acetate, polystyrene, polyvinyl chloride or polyethylene terephthalate is used. As the image receptor, a properly selected printing sheet such as paper or a film is used.
As an apparatus for preparing the above prepress proof, generally, there has been proposed an image-transfer apparatus having a platen to which an image-forming material is to be attached and a transfer cylinder to which an image receptor is to be attached. The platen and the transfer cylinder are brought into contact so that a photosensitive layer of the image-forming material and an image receptor are brought into contact with each other under pressure to transfer an image formed on the photosensitive layer to the image receptor in a nip portion formed by the platen and the transfer cylinder. The image receptor, such as paper, is preliminarily heated, and the image formed on the photosensitive layer of the above image-forming material is transferred to the paper by brining the photosensitive layer into contact therewith under pressure, whereby the transfer of the image proceeds smoothly to give an aesthetically fine printed sheet. Generally, therefore, a means for heating the surface of the transfer cylinder at a proper temperature, e.g., around 100.degree. C., is provided within the transfer cylinder to which the image receptor is to be attached.
The image receptor used in the present invention, such as paper, has the property of undergoing curling or undulation under heat. Therefore, the image receptor is liable to easily curl or undulate when it is brought into contact with the hot transfer cylinder surface. For holding the image receptor with one set of a holder and a holder seat extending in the axial-length direction of the transfer cylinder there is conventionally used a device for holding the bottom end side of the image receptor, with the image receptor is inserted into a narrow gap provided between the holder and the holder seat so that the portion of the image receptor which is to be held by the holder undergoes curling, undulation and shrinkage. It is therefore difficult to hold the image receptor with the holder firmly, and the attaching of the image receptor is liable to be defective. Further, it has not been possible to detect whether the image receptor is firmly attached by means of a device for attaching its top end side and a device for attaching its bottom end side in case of the above defective attaching. It has therefore been necessary to press the bottom end side of the image receptor manually and uniformly to a cut-off portion between the transfer cylinder surface and the holder before the bottom end side is held by the holder and holder seat of the device (also to be referred to as "clamp" hereinafter) for holding the bottom end side. Further, the transfer cylinder is heated for heating the image receptor at a proper temperature, e.g., around 100.degree. C. In this case, the hand is likely to touch the transfer cylinder surface often when the image receptor is attached to the transfer cylinder surface. There is therefore a safety problem. There is also another problem in that manually pressing the bottom end side of the image receptor to the cut-off portion between the transfer cylinder surface and the holder seat increases the number of steps and requires additional time in the transfer operation.
Further, since the transfer cylinder has a relatively high temperature, there is a risk of the hand being burned when the image receptor is taken out from the transfer cylinder. There is another risk of the fingers being caught in the means for holding the image receptor to the transfer cylinder, such as a clamp. Moreover, when the image receptor is manually ejected, it means an additional step is required, and the time required for the transfer operation increases.
As an image receptor attaching means, it has been hitherto strongly desired to develop an image receptor holding device and a method for ejecting the image receptor from the transfer cylinder, with which a soft sheet-like substance such as the image receptor can be attached and detached easily and the image receptor can be held without causing a failure in attaching and without producing a defective print product. However, no device has yet been developed which can satisfy the above requirements.
It is therefore an object of the present invention to provide an image transfer apparatus for producing a prepress proof which has an image receptor attaching device with which a soft sheet-like substance such as the image receptor can be easily attached and detached and the image receptor can be attached while keeping the hand almost away from the transfer cylinder surface.
It is another object of the present invention to provide a method for ejecting the image receptor which permits the ejection of the image receptor from the transfer cylinder while keeping the hand almost away from the transfer cylinder and without impairing the quality of the image transfer product.